1. Field of the Invention
This invention relates to chain saw guide bars and in particular it relates to an improved tooth configuration of an idler sprocket in the nose of the chain saw guide bar.
2. Background Information
Motorized chain saws have an elongate guide bar on which a saw chain travels. The bar is attached to a side of the housing of the chain saw power head with one end of the bar aligned with a drive sprocket that is driven by the drive shaft from the power head. The opposite end of the guide bar (referred to as the nose end) is generally semi-circular and it blends the top edge of the bar to the bottom edge.
The drive sprocket propels (pulls) an endless articulate saw chain around the guide bar. This occurs as a result of the sprocket teeth engaging depending portions of the saw chain referred to as drive tangs located on the center links of the saw chain (center drive links). The saw chain has, along its length, a plurality of alternating right and left hand cutter links that are side mounted in reference to the center drive links. Each cutter link is pivotally attached (by a pin or rivet) to a right or left side of the drive links. A tie strap link, having the same lower body configuration as the cutter link, is provided on the side of the drive links opposite the cutter links. The tie strap and cutter links, all being attached to the left or right hand side of the drive links are referred to collectively as side links.
Accordingly the saw chain sequence typically consists of alternating center drive links and interconnected pairs of side links, the side links being paired cutter links and tie strap links (a full house chain) or every other pair of side links may be paired tie strap links to space the cutters along the chain length (skip tooth chain). The center drive links travel in a peripheral groove in the guide bar to guide the saw chain as it travels around the bar. The pair of side links (a cutter link and a tie strap or a tie strap pair) of the saw chain ride in frictional contact on the side rails (edges) of the groove and thus provide the necessary bearing support for the chain during the cutting action. The bottom edge of the cutter links and the tie straps are notched between their pivotal mounting points to approximate a fit to the changing bar contour and to accommodate certain types of drive sprockets.
The highest rate of wear occurs at the nose section of the bar as the chain travels from the upper edge of the bar in a curvilinear path to the lower edge of the bar. To alleviate the wear rate and to aid in transporting the chain around the nose section, an idler sprocket or nose sprocket may be rotatably mounted in the nose of the bar. The nose sprocket is rotatable about an axis slightly forward of the center of the radius of the bar nose to smoothly engage and lift the saw chain. The nose sprocket has a different function and is not to be confused with the drive sprocket. The nose sprocket is propelled by the drive tangs of the saw chain's center drive links engaging the teeth of the nose sprocket. The sprocket is so positioned in the nose of the bar that it lifts the side links of the chain slightly off the rails as the chain traverses the nose of the bar.
In a conventional nose sprocket guide bar, each tooth of the sprocket enters between adjoining (front to back) drive links, with the tip of the tooth extending into the chassis of the chain. That is, the tip of the tooth is positioned between laterally opposite side links with the tooth tip extending above the top of the notch in the side links. This was heretofore thought to be a desirable feature to provide stability to the saw chain as it traveled around the bar nose.
The nature of the alternating right and left hand cutter links is such that in a cutting operation they exert lateral forces on the chain. These lateral forces tend to twist the saw chain as well as to laterally urge the saw chain out of its intended path. (The twisting and lateral forces are hereafter collectively referred to as "lateral forces".) In normal cutting conditions the drive tangs of the center drive links are entrained in the bar groove so that the engagement of the drive tangs with the groove walls absorbs the lateral forces. When the nose section of the bar is in the cut, the lateral forces are at least partially applied to the portion of the sprocket teeth tips projected between the side links. Sprocket teeth and bearings are unable to withstand these lateral forces and undue wear or breakage of the sprocket is common.
The lateral forces can also shove the saw chain and the sprocket teeth out of alignment at the point where the saw chain starts to mount the sprocket. The tip of the sprocket tooth in such an instance may then contact the bottom of a side link, lifting the chain further than intended so as to raise the drive tangs out of the bar groove. This condition can result in the chain being thrown off the guide bar. This can result in further damage to the chain saw and saw chain and may be a safety risk to the operator.
It was found that chamfering the tip of the sprocket aided the tip to "find" the opening between the side links for centering the chain on the bar nose. However, it was found that on occasion the chain and the sprocket could still get sufficiently out of alignment that the tip of the tooth, although chamfered, would come into contact with the bottom edge of a side link to cause the chain to jump off the bar as described.
The chamfering of the sprocket was an added operation during manufacture and of course added to the cost. The problem of the chain jumping off the bar because of the sprocket tooth tip engaging the bottom edge of the side link was reduced but not eliminated, and the problem of the sprockets having to bear the severe lateral forces during a nose cutting operation still resulted in rapid wear and breakage of the nose sprocket.